1. Field of the Invention
Implementations of various technologies described herein generally relate to seismic data processing.
2. Description of the Related Art
In a typical land or marine seismic survey, seismic receivers are installed in specific locations around an area of the earth in which hydrocarbon deposits may exist. Seismic energy sources, such as vibrators or air guns, may move across the area and produce acoustic signals, commonly referred to as “shots,” directed down to the earth, where they are reflected from the various subterranean geological formations. The shots typically generate seismic waves, which are detected by the seismic receivers. The seismic waves propagate into the formations in the earth, where a portion of the waves reflects from interfaces between subterranean formations. The amplitude and polarity of the reflected waves are determined by the differences in acoustic impedance between the rock layers comprising the subterranean formations. The acoustic impedance of a rock layer is the product of the acoustic propagation velocity within the layer and the density of the layer. The seismic receivers detect the reflected seismic waves and convert the reflected waves into representative electrical signals. The signals are typically transmitted by electrical, optical, radio or other means to devices which record the signals. Through analysis of the recorded signals (or traces), the shape, position and composition of the subterranean formations can be determined.
Seismic waves, however, reflect from interfaces other than just those between subterranean formations, as would be desired. As such, seismic data may often include multiples. Multiples refer to seismic energy that has been reflected downwards at least once before it has been received by the seismic receivers. Free-surface multiples include seismic energy that has been reflected downward from the free-surface. In a land seismic survey, the free-surface is the upper surface of the earth. In a marine seismic survey, the free-surface is the surface of the body of water. Internal multiples include seismic energy that has been reflected downward from a reflector below the free-surface before it is received by seismic receivers. In a marine seismic survey, seismic waves also reflect from the water bottom and the water surface, and the resulting reflected waves themselves continue to reflect.
Waves which reflect multiple times in the water layer between the water surface above and the water bottom below are called “water-bottom multiples”. Water-bottom multiples have long been recognized as a problem in marine seismic processing and interpretation, so multiple attenuation methods based on the wave equation have been developed to handle water-bottom multiples. However, as described above, a larger set of multiples containing water-bottom multiples as a subset can be defined. The larger set includes multiples with upward reflections from interfaces between subterranean formations in addition to upward reflections from the water bottom. The multiples in the larger set have in common their downward reflections at the water surface and thus are called “surface multiples”.
There are many methods of removing surface multiples from seismic data. One common method uses a prestack inversion of a recorded wavefield to remove all orders of all surface multiples present within the seismic signal. As used herein, the term “surface-related multiple attenuation” refers to that method. Unlike some wave-equation-based multiple-attenuation algorithms, surface-related multiple attenuation does not require any modeling of or assumptions regarding the positions, shapes and reflection coefficients of the multiple-causing reflectors. Instead, surface-related multiple attenuation relies on the internal physical consistency between primary and multiple events that must exist in any properly recorded seismic data set. Therefore, the information needed for the surface-related multiple attenuation process is already contained within the seismic data.
Although theoretically surface-related multiple attenuation algorithms may be used to remove all orders of all surface multiples present within the a seismic signal, in practice, two difficulties arise. First, because a direct inversion of seismic data is difficult, the process is typically approximated by computing terms of a truncated series expansion. As a result, the surface multiples in a seismic data set are merely approximate predictions. Second, because the seismic field data sets are not usually sampled well enough to predict surface multiples properly, the data set must be interpolated and extrapolated in order to make it more suitable for calculation. Due to these difficulties, predicted multiples contain errors. Such errors can be accommodated by adaptively subtracting the predicted multiples from the seismic data set. However, it has generally been observed that the final results are better when the adaptive subtraction has to accommodate smaller errors in the predictions.